This invention relates to improvements in focused ultrasonic transducers, and more particularly to an ultrasonic transducer providing efficient energy transfer without defocusing the ultrasonic beam.
To couple focused ultrasonic energy into an interrogated object having a relatively flat surface, it is conventional to employ a piezoelectric crystal having a concave active surface and a filler such as mica-loaded epoxy, between the active surface and the object. The filler has a convex surface and a flat surface through which the ultrasonic energy is coupled from the crystal to the object. The filler has an acoustical impedance between that of the crystal and that of the object to provide an impedance match, but has a large sonic velocity relative to water. As a result of the large sonic velocity, when the interrogated object is water or body tissue, the filler defocuses the coupled ultrasonic energy. Consequently, a shorter curvature must be formed on the concave active surface to compensate for the defocusing effect, which makes manufacturing more difficult.